The study of motor neuron differentiation in the developing spinal cord has provided a model system for defining the cellular interactions and molecular nature of inducing factors that control neuronal identify in the vertebrate nervous system. The source and origin of signals that control the diversification of motor neuron subtypes however remains unclear. In this project we will focus on the control of motor neuron diversity in the lateral motor column (LMC), a class of motor neurons that innervates target muscles in the limb. In preliminary studies we have provided evidence that three distinct signaling molecules that have been implicated in cellular differentiation and transformation: retinoids, ETS domain transcription factors and receptor tyrosine kinases of the Eph family are expressed by subsets of motor neurons in the LMC. These studies suggest that the analysis of motor neuron subtype diversity in the LMC may provide novel information on the function of classes of molecules implicated more generally in cellular differentiation and oncogenesis. Using a combination of in vitro assays of motor neuron differentiation and molecular genetic manipulations of gene expression in motor neurons in mouse embryo we will address three main issues: 1. The role of retinoid signaling in the specification of motor neuron columnar subtype identity within the LMC. 2. The role of ETS transcription factors in specifying the identity of motor neuron pool identity within the LMC. 3. The role of Eph kinases and their Ephrin ligands in the organization of motor neurons in the LMC and in the projection of specific subsets of motor axons to their targets in the limb.